The MPC says that '66 hoses were 1/2" longer than '65 hoses on 6-cylinder cars. I believe that Ford was seeing failures and increased the length to reduce some of the stress. I have an idea why these hoses are different lengths for 1966 6-cylinders:
I found a reference that talked about the length of aftermarket hoses and the relationship of rubber to metal tubing. It seems that if the rubber portion is too short and it is forced into a helical steering movement it puts more stress on the rubber and crimp connection; possibly causing premature failure.
Here's my reasoning:
6-cylinder cars had a slightly smaller front wheel track than 8-cylinders, 55.4" to 56" respectively. Also when the inside wheel was turned to 20 degrees the outside wheel of a power steering 6-cylinder was at a sharper 20-1/8 degrees, while the 8-cylinder was at 18-3/4 degrees. I don't believe the turning radius of both models were the reported 38.90 feet. The 6-cylinder likely had a sharper turning radius given the above specifications, and Ford realized they had to lengthen the hose for the '66 model year to adjust for the stress being put on these hoses by the tighter turning radius.
Thoughts?